The present invention relates to a dielectric medium for capacitor of semiconductor device, and particularly to a dielectric medium made from a (Sr,Ba)Nb.sub.2 O.sub.6 solid solution and for use in the capacitor a layer of high density devices such as a dynamic RAM or a static RAM.
Conventionally, in an effort to improve the density of the high density devices such as DRAM or SRAM (Dynamic RAM or Static RAM), the capacitor area is expanded by forming the capacitor in a stacked or trenched structure. However, there is a limit in improving the density by expanding the capacitor area by the above mentioned method.
Further, in the conventional capacitor layer of DRAM or SRAM, dielectric materials in the form of an oxide layer (SiO.sub.2) or a nitride layer (Si.sub.3 N.sub.4) are used. However, the dielectric constants of such dielectric materials are respectively 3.8 and 7, and therefore, the use of such materials is encountered with a great difficulty in improving the density and reliability of the semiconductor devices, because the capacitor layer becomes thinner and thinner as the density of the semiconductor device is increased.
If such a problem is to be solved, there is required a ferro-electric material or a high dielectric material rather than the above described oxide layer or the nitride layer. In this connection, Ta.sub.2 O.sub.5, TiO.sub.2 and Nb.sub.2 O.sub.5 are proposed as the high dielectric material, while Pb(Zr,Ti)O.sub.3 series such as (Pb,La)(Zr,Ti)O.sub.3 are proposed as the ferro-electric material.
These ferro-electric materials and high dielectric materials have high dielectric constants, and therefore, they are advantageous for improving the density. However, their dielectric loss coefficients are as high as over 1%, and therefore, there is the problem that the leakage current is increased. Further, there occurs a phase transformation such that the crystal structure is changed at the Curie temperature during the heat treating process, and thus, the thin dielectric layer is destroyed due to the temperature variation. Therefore, the ultimate result is that there is a problem in using the ferro-electric material or the high dielectric material for the dielectric layer of capacitor.
There is a further problem such that, of the ferro-electric materials, the Pb(Zr,Ti)O.sub.3 series causes destruction of stoichiometry due to the volatile component PbO.